1. Field of the Invention
The present invention relates to a magneto-optical recording medium and a method for preventing erasure of the information recorded on the magneto-optical recording medium, and more particularly to a recording medium having a vertical magnetization film onto which the information is recorded and read out by an optical means such as a laser beam, and a method for preventing the erasing of the information recorded on this type of recording medium.
2. Description of the Prior Art
In recent years, recording systems have been proposed wherein the information is recorded and played back by effectively utilizing the magneto-optical characteristics of an amorphous (noncrystalline) alloy material which is used as the recording medium.
This is due to the fact that the amorphous alloy thin film is a "vertical magnetization film" in which the direction of magnetization is perpendicular to the surface of the film.
Recording of the information on this amorphous thin film takes place in a manner such that the orientation or the direction of magnetization of the desired portions of the amorphous alloy film, which is initially magnetized in a preselected direction, is turned over by heating those portions above the Curie temperature or a compensation temperature under the condition of application of a weak external magnetic field.
More specifically, the amorphous alloy thin film which is previously downwardly (corresponding to the binary "0" signal) magnetized is effected by a weak upward bias magnetic field and only the portions of the thin film on which the "1" signal is to be recorded are applied with the laser beam so as to raise the temperature of those portions above the Curie temperature or a compensation temperature to cause the change in the direction of magnetization.
On the other hand, the reading out of the information recorded on the thin film takes place by utilizing the magnetic Kerr effect which is an interaction between the magnetism of substance and the light beam.
When a linearly polarized laser beam is reflected by the magnetized thin film, the plane of polarization is rotated in a direction which is determined by the direction of the magnetization.
Therefore, the information recorded on the amorphous thin film is read out by detecting the direction of rotation of the plane of polarization of the reflected laser beam by means of an analyzer.
FIG. 1 shows a schematic illustration of an example of this type of record and playback system using a magneto-optical recording disc. With such a system, utilizing the vertical magnetization of thin film, it is advantageous that the information can be rewritten, which is referred to as "reversible recording". This type of system, however, has suffered from the problem that the recorded information may erroneously or unintentionally be erased.